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I recently moved my small machine shop to a new location. Both locations had 3-phase power, and I wired up a 3-phase air compressor in both locations.

In both locations, I used the convention of black-red-blue wiring on L1-L2-L3 bus bars in the electrical panel.

But in the new location, the motor is running backward, indicating reversal of two of the wires (or that the previous location was “incorrect”).

I can't find anything online that says L2 should lag L1 by 120°, and L3 should lag L2 by 120°, but is that the convention? Are the mains leading into my panel (new or old) mis-wired? Or did I just make a mistake somewhere?

My specific question: what is the required phase relationship between L1, L2, and L3 (if any)?

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    \$\begingroup\$ I was wondering about this myself. Watching for an answer. \$\endgroup\$ – mkeith Nov 10 '17 at 20:40
  • \$\begingroup\$ Each country has their own colour codes, and while in theory this should work well, in practice it is so often messed up that there is a good market for rotation direction testers and adapter plugs to change rotation direction. I even have a few 32A and 63A plugs where inside the plug you can change the direction (upload.wikimedia.org/wikipedia/commons/thumb/0/0b/…) \$\endgroup\$ – PlasmaHH Nov 10 '17 at 22:43
  • \$\begingroup\$ I'm not asking about the color codes, but rather the phase relationship. \$\endgroup\$ – Rick Nov 11 '17 at 0:09
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They sell testers for this purpose starting at < $20- sometimes running in reverse can cause severe damage to machinery.

enter image description here

I would not trust that the convention was followed even if there is one, in such a case.

I also cannot recommend the cheap testers, especially on > 240VAC due to potential for arc flash and such like- a real and appropriate CAT number should be present. Something from a reputable maker might be more like $150 (the one shown above is of the former category and is sold on Amazon- meaning that they probably are required to carry enough liability insurance to cover your next of kin).

Older testers actually had a small 3-phase motor and you would observe the direction of rotation, but newer ones are generally electronic.


Directly in answer to your question, the color code is listed as "common practice" in the US and "mandatory" in Canada as follows (somewhat unofficial source here):

enter image description here

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  • \$\begingroup\$ I expect the colours "red, blue and yellow" - what's green? \$\endgroup\$ – Solar Mike Nov 10 '17 at 21:35
  • \$\begingroup\$ @SolarMike Yellow, Green and Red are Chinese standard for 3-phase. Green + yellow stripe is ground. I said it was a cheap one. ;-) \$\endgroup\$ – Spehro Pefhany Nov 10 '17 at 21:44
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    \$\begingroup\$ I'm not asking about the color codes. I'm asking about the actual phase relationship between L1, L2, and L3. \$\endgroup\$ – Rick Nov 11 '17 at 0:08
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    \$\begingroup\$ @Rick, I don't think the tester is inspecting the color of the insulation. It must be making a phase angle comparison. It is true that Spehro did not specifically answer the question, but his answer implies that there is in fact a correct phase relationship between the three phases, and an incorrect one. He just didn't say if L2 leads or lags L1 by 120 degrees. I assume it lags because that is how the pictures are usually drawn. \$\endgroup\$ – mkeith Nov 11 '17 at 2:52
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    \$\begingroup\$ L1 followed by L2 followed by L3 is the conventional order, but if you're thinking you can depend on that, that would not be a prudent approach. If you already damaged something you may have trouble fixing blame on someone else. \$\endgroup\$ – Spehro Pefhany Nov 11 '17 at 6:04
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It could be, that wiring in old location was wrong, or the new one has wrong phase sequence. If all machines were ready to operate on old installation, then maybe the easiest way is to swap two phase conductors in the new installation, no matter which installation is correct.

This is not an issue, there are three phase plugs, that have two moving rods. When you get on place, you turn the machine on, see if it rotating correctly, if not twist the rods inside the connecting plug. See, you're not alone with those kind of problems.

enter image description here

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    \$\begingroup\$ The question was, "Is there a convention for the ordering of phases in three-phase power in the US?", not "How do I reverse a motor?". \$\endgroup\$ – Transistor Nov 11 '17 at 8:56
  • \$\begingroup\$ Agreed, it could be the old location was incorrect. I just wanted to know if there was a convention for the service entrance. \$\endgroup\$ – Rick Nov 14 '17 at 2:04

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